The invention relates to a process for the manufacture of semi-finished products or preformed parts each having high thermal creep-resistance and each made from sintered or molten fabricated materials of dispersion-strengthened alloys. The alloy materials are made up of the refractory metals vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, either alone, or in combination with one another, or as a major constituent with other metal components.
For semi-finished products, and in particular for preformed parts made of refractory metals, there is a need for improved thermal stability characteristics, primarily higher resistance to thermal creep. The stability characteristics of such metals can be achieved by alloying, deformation strengthening, age-hardening processes, and dispersion hardening. Among the processes for the manufacture of creep-resistant alloys, doping and reshaping have proven quite effective in creating a stacking structure in the metal, that is, a structure in which the individual metal crystals exhibit a minimum aspect ratio of 1:2.
For a long period of time, refractory metals were doped primarily with potassium, aluminum, and silicon for this purpose. In recent years, doping with oxide-and carbide-based dispersoids has acquired increased significance. Such alloys are described, for example, in Austrian Patent Specification 386 612.
Of the processes known in the art for the manufacture of materials resistant to thermal creep, thermal reshaping, which is implemented by immediately successive and the largest reshaping steps possible at very high deformation strains, i.e., 90% and more, yields the best thermal creep-resistance values. During this process, the reshaped materials are subjected to final recrystallization annealing to form as distinct a stacking structure as possible. Those processes which involve multiple reshaping steps and annealing operations are complex and expensive, but according to prevailing technical wisdom are unavoidable in order to achieve optimum thermal creep-resistances.
Alternatively, thermal reshaping with up to 60-90% deformation, is achieved in a single operation with intermediate heating of the workpiece, if necessary. If, for example, the reshaping process cannot be implemented to the desired degree of deformation, or the alloy cannot be reshaped quickly enough to the desired shape without cooling off to an excessive degree, then the thermal creep-resistance values of the alloys fabricated in this manner are markedly lower than those values achieved when a stacking structure is formed.